1. Field of the Invention
The present invention relates to a method for making a polycrystalline thin film having a highly oriented grain structure and a method for making oxide superconductor on such a thin film base and an apparatus for making said polycrystalline thin film.
2. Description of the Related Art
Oxide superconducting materials discovered in recent years are excellent super-conductors having a critical temperature higher than the liquid nitrogen temperature, but many problems remain to be resolved before such oxide superconducting materials can be used as practical superconductor devices. One such problem is that the critical current density is low for these oxide superconductors.
The problem of low critical current density is caused largely by the anisotropic electrical properties of the crystals in the superconductor themselves, and especially, it is known that an electrical current can flow relatively easily in the directions of a- and b-axes but has a difficulty flowing in the c-axis direction. Therefore, to deposit an oxide superconductor on a substrate base and to use such a material as a superconducting device, it is necessary to form an oxide superconducting layer on a substrate base whose grain structure has a highly-developed controlled orientation, and furthermore, the a- and b-axes must be made to align in the direction of the current flow while the c-axis is oriented in another direction which does not impede the current flow.
Various methods have been tried in the past to deposit oxide superconductors of a suitable orientation on a substrate such as plate and metal tape. One such method currently in use is a thin film growth technique based on sputtering of superconducting oxide material on a single crystal substrate base of a material such as MgO or SrTiO.sub.3 which have similar crystal structures to oxide superconductors.
Using such a single crystal substrate material to grow a thin film by sputtering, for example, it is possible to deposit a single crystal layer having an excellent directionality in the crystal orientation, and it is known that a high critical current density exceeding several hundred thousand of amperes per square centimeter can be achieved in the superconducting oxide layer formed on the single crystal base.
To use the oxide superconductor as an electrical conductor, it is necessary to deposit a uniformly oriented superconducting layer on a longitudinally extending base, for example a tape base. However, when such a layer is deposited on a metal tape, because the substrate metal itself is a polycrystalline material and its crystal structure is quite different from that of the oxide material, it is virtually impossible to produce a highly oriented superconducting layer. Additionally, because of heat treatments necessary to develop superconducting properties, diffusional reactions which can occur between the superconducting oxide layer and the metal tape base disturb the interface structure and degrade the superconducting properties.
For these reasons, general practice is to form a superconducting layer on top of a sputtered intermediate layer, comprised by materials, such as MgO or SrTiO.sub.3, on a metal tape. However, the problem with a superconducting oxide layer formed on such an intermediate layer is that it exhibits only a low critical current density (for example, several thousand to tens of thousand A/cm.sup.2). This problem is thought to be due to the following causes.
FIG. 15 shows a cross sectional view of a sputtered superconducting oxide layer 3 formed on top of an intermediate layer 2 on a base 1 of a metal tape, for example. The superconducting oxide layer 3 is a polycrystalline layer and is comprised by numerous randomly oriented grains 4. Close examination of the individual grains 4 reveals that although the c-axis of each grain 4 is at right angles to the base, both a- and b-axes are oriented in random directions.
When the a- and b-axes are randomly oriented in the neighboring grains, quantum coupling in the superconducting state is destroyed at the grain boundaries which are irregular lattice structures, and the result is that the superconducting properties, especially the critical current density become seriously affected.
Also, because the underlying intermediate layer 2 is polycrystalline without the uniform orientation of a- and b-axes, the superconducting oxide layer 3 becomes a polycrystalline layer of randomly oriented a- and b-axes, and the growth of the layer 3 occurs in conformity with the underlying random orientation nature of the intermediate layer 2.
Technology of growing an oriented film of various materials on polycrystalline substrate is utilized in fields other than the above-mentioned oxide superconductor field. For example, they are useful in optical thin films, opto-magnetic discs, circuit boards, high frequency waveguides and signal filters, as well as in cavity resonators, but in every field, an important requirement is to produce a polycrystalline film having a highly developed crystal orientation of a uniform quality. In other words, the quality of the thin film for optical, magnetic and circuit applications would be expected to be better if the film can be formed on a polycrystalline base having a controlled grain orientation, and it would be even more desirable if a properly oriented films for such applications can be deposited directly on the substrate base.
For these reasons, the present inventors have been investigating processes of forming a polycrystalline layer of yttrium-stabilized zirconia (abbreviated to YSZ hereinbelow) on a metal tape and subsequently depositing superconducting oxide layer on the polycrystalline layer to produce an oxide superconductor of superior properties.
These efforts have resulted in publications of the following patent applications, for producing a polycrystalline film of a controlled orientation and oxide superconductors formed thereon: JPA, First Publication, H4-329865 (Application No. H3-126836); JPA, First Publication, H4-331795 (Application No. H3-126837); and JPA, First Publication, H6-145977 (Application No. H4-293464).
These studies have shown that irradiating ion beams at an inclined angle to the YSZ layer being formed enabled to obtain a superior orientation control of the grains.
Concurrent with these investigations, studies have been conducted on how to produce polycrystalline thin films and oxide superconductors on an extending or large area substrate. As a result of the accumulated efforts, not only a method of making polycrystalline thin films to provide a superior control over the crystal orientation but also a method of forming an oxide superconductor of superior superconducting properties on top of such a substrate base have been developed.